The invention relates to a method of using an acid flux for lowering the melting point of a lime-silica slag. More particularly, the invention relates to a method of using an acid flux for lowering the melting point of a blast furnace slag for the production of steel from molten carbon saturated iron.
It is well known the primary charge material for steel manufacturing in an oxygen refining furnace is molten iron smelted in a blast furnace. The charge material may also include various fluxing agents and cold ferrous scrap. Low carbon steels for deep drawing applications such as automotive parts must have very low levels of impurities. For example, the steel specification for sulfur may be as low as 0.010% by weight. However, molten iron from a blast furnace frequently includes as much 0.03% by weight or more sulfur. Accordingly, sulfur may have to be removed from the molten iron prior to refining or from the steel after refining. As is well understood by those skilled in the art, the preferred practice of desulfurization is to remove sulfur from the molten iron prior to charging the molten iron into a steel refining furnace. For the later type of desulfurization, molten iron from a blast furnace may be cast into a transfer car such as a torpedo car. Blast furnace slag, entrained with the molten iron during the cast at the blast furnace, forms an insulative cover over the molten iron in the transfer car. The molten iron then is transported to a desulfurization station where a desulfurization agent may be injected into the transfer car or the molten iron may be poured first into a transfer pot such as a ladle with the desulfurization agent being injected into the molten iron while in the transfer pot. In any event, sulfur removed from the molten iron by the desulfurizing agent is retained in the slag and the slag must be removed to prevent sulfur from reverting back to the steel during refining. Otherwise, additional desulfurization may be required during or after refining.
For various reasons, the temperature of the molten iron in the transfer pot may be below that necessary for the slag to be in a fluid condition. Even if desulfurization of the molten iron is not required, the blast furnace slag is ladened with sulfur removed during smelting and should not be transferred to the steel refining furnace. This slag normally is removed from the transfer pot by racking or skimming from the upper surface of the molten iron. If the slag has solidified or has insufficient fluidity, it cannot be removed. For molten iron requiring desulfurization, a cold slag is even more likely because a temperature drop of 30.degree. F. (17.degree. C.) or more in the molten iron can occur while injecting the desulfurization agent. More importantly, desulfurization agents normally contain lime. Adding additional lime may further raise the melting point of the slag.
It is well known to use a flux including a fluoride ion (F.sup.-) source to increase slag fluidity by lowering the melting point of the slag. Fluoride containing materials for this purpose include fluorspar, cryolite and sodium fluoride.
It is also well known to include soda based materials in a flux to not only increase slag fluidity but also to remove impurities such as phosphorus and sulfur from molten metals. Soda based materials for these purposes include soda ash, sodium silicate, borax and cryolite.
Nevertheless, there remains a need to further reduce the melting point of a lime-silica type slag requiring a high sulfur capacity. More particularly, a process is needed to increase the fluidity of a lime-silica slag by adding a flux to a molten metal forming a flux treated slag having good chemical stability, good refining removal characteristics, and causing minimal erosion to vessel refractory linings. For example, basic slag after desulfurization of carbon saturated molten iron may contain 40% by weight or more lime. These slags tend to solidify at molten iron temperatures of about 2300.degree. F. (1260.degree. C.) even though the iron solidifies at a temperature of about 2125.degree. F. (1162.degree. C.). Low fluidity slags have diminished dephosphorization and desulfurization characteristics and are difficult to remove from the molten metal. Accordingly, there remains a need to fluidize lime-silica type slags especially in the temperature range of 1162.degree.-1260.degree. C. so that they can be removed from the molten metal using mechanical skimming equipment, have good refining characteristics especially for sulfur and phosphorus, have high sulfur capacity, have good chemical stability, and case minimal erosion to refractory linings. We have determined for satisfactorily reducing the melting point of lime-silica slags it is necessary to maintain a proper balance of fluoride ion and sodium oxide in the final slag composition. More particularly, for slags containing a high lime content, it is necessary to maintain a proper balance between fluoride ion, and the oxides of sodium, calcium, silicon and aluminum.